Steel sheet for warm press forming, warm-pressed member, and manufacturing methods thereof

ABSTRACT

Provided is a warm-pressed member comprising, by weight %, C: 0.01% to 0.5%, Si: 3.0% or less (excluding 0%), Mn: 3% to 15%, P: 0.0001% to 0.1%, S: 0.0001% to 0.03%, Al: 3.0% or less (excluding 0%), N: 0.03% or less (excluding 0%), and the balance of Fe and inevitable impurities. After a warm press forming process and a cooling process, the warm-pressed member has a microstructure comprising: 5 volume % to 50 volume % of retained austenite; and at least one of ferrite, martensite, tempered martensite, and bainite as a remainder.

TECHNICAL FIELD

The present disclosure relates to a steel sheet for automobilestructural members or reinforcement members, and more particularly, to asteel sheet that may be increased in strength, elongation,shock-absorbing ability, and plating corrosion resistance after a warmpress forming process. In addition, the present disclosure relates to awarm-pressed member formed of the steel sheet, and methods ofmanufacturing the steel sheet and the warm-pressed member.

BACKGROUND ART

Automobiles are increasingly required to have high fuel efficiency andcrashworthiness in order to protect both the environment and automobilepassengers. Thus, a great deal of research has been conducted to developlightweight and crashworthy automobiles using high-strength chassis.

For example, hot pressing methods have been proposed to producehigh-strength steel sheets improved in terms of formability and shapecontrollability. Such methods are disclosed in Patent Documents 1 and 2.In such methods, a steel sheet having a single phase of austenite thatis low in strength but high in formability is subjected to a heattreatment process and a pressing process, and is then rapidly cooled bydies. Therefore, ultra-high-strength final products having martensite asa main microstructure phase are manufactured.

However, since a steel sheet having a single phase of austenite isheated at high temperature in the methods, oxide scale may have to beremoved from the surfaces of the steel sheet after the heat treatment ifthe steel sheet is not a plated steel sheet, and high costs may beincurred in heating the steel sheet to a high temperature.

If Zn-plated or Al-plated steel sheets are processed by the methods,plating materials may be evaporated or fused to cause a decrease inproductivity. Since the melting point of zinc (Zn) is 500° C. or lessand the melting point of aluminum (Al) is lower than 700° C., if a steelsheet plated with zinc (Zn) or aluminum (Al) is heat-treated at hightemperature as described above, the zinc (Zn) or aluminum (Al) may bepartially melted and thus may not properly function as a platingmaterial. In addition, the zinc (Zn) or aluminum (Al) may be fused todies or forming machines to deteriorate the formability of the steelsheet.

Furthermore, although the strength of a steel sheet is increased throughsuch a high-temperature forming process, the elongation of the steelsheet is reduced to lower than 10% because 90% or more of themicrostructure of the steel sheet is formed by martensite, and thus thesteel sheet may not have sufficient crashworthiness. Therefore, thesteel sheet may only be used to manufacture limited kinds of automotivecomponents.

(Patent Document 1) Korean Patent Application Laid-open No. 2007-0057689

(Patent Document 2) U.S. Pat. No. 6,296,805

DISCLOSURE Technical Problem

An aspect of the present disclosure may provide a steel sheet for warmpress forming having high strength, good elongation, and thus improvedcrashworthiness after being warm pressed, and a member formed bywarm-pressing the steel sheet.

An aspect of the present disclosure may also provide a plated steelsheet for warm press forming that can have good corrosion resistanceeven after a heat treatment such as a heat treatment of a warm pressforming process, and a warm-pressed member.

Technical Solution

According to an aspect of the present disclosure, a steel sheet for warmpress forming may include, by weight %, C: 0.01% to 0.5%, Si: 3.0% orless (excluding 0%), Mn: 3% to 15%, P: 0.0001% to 0.1%, S: 0.0001% to0.03%, Al: 3.0% or less (excluding 0%), N: 0.03% or less (excluding 0%),and the balance of Fe and inevitable impurities.

According to another aspect of the present disclosure, a method ofmanufacturing a steel sheet for warm press forming may include: heatinga steel slab to a temperature within a temperature range of 1000° C. to1400° C., the steel slab including the above-mentioned composition ofthe steel sheet; forming a hot-rolled steel sheet by performing a hotrolling process on the steel slab and then a finish-rolling process onthe steel slab at a temperature within a temperature range of Ar3 to1000° C.; and coiling the hot-rolled steel sheet at a temperature higherthan Ms but equal to or lower than 800° C.

According to another aspect of the present disclosure, a warm-pressedmember may include the above-mentioned composition of the steel sheet,wherein after a warm press forming process and a cooling process, thewarm-pressed member may have a microstructure formed by: 3 volume % to50 volume % of retained austenite; and at least one of ferrite,martensite, tempered martensite, and bainite as a remainder.

According to another aspect of the present disclosure, a method ofmanufacturing a member by warm press forming may include: performing awarm press forming process on a steel sheet including theabove-mentioned composition of the steel sheet; and cooling the steelsheet, wherein the warm press forming process may include a heattreatment process including: heating the steel sheet to a temperaturewithin a temperature range of Ac1 to Ac3 at a heating rate of 1° C./secto 1000° C./sec; and maintaining the steel sheet at the temperaturewithin the temperature range for 1 second to 10000 seconds.

Advantageous Effects

The present disclosure relates a method of manufacturing anultra-high-strength steel sheet that can be used for manufacturingstructural members, reinforcement members, and shock-absorbing membersof automobiles, and a member formed by warm-pressing the steel sheet.According to the method of the present disclosure, a steel sheet havinga ultra-high tensile strength of 1000 MPa or greater and good elongationafter a heat treatment of a warm press forming process can bemanufactured, and a heat-treatment member formed of the steel sheet canbe provided. That is, according to the present disclosure, theapplication of a heat treatment type ultra-high-strength steel sheet canbe extended to impact members.

DESCRIPTION OF DRAWINGS

FIG. 1 is a graph illustrating a thermal history of a hot press formingprocess of the related art.

FIG. 2 is a graph illustrating a thermal history of a warm press formingprocess of the present disclosure.

BEST MODE

In the present disclosure, the term “warm press forming” refers toforming a steel sheet to have a certain shape after heat-treating thesteel sheet at a temperature equal to or lower than the austenite singlephase region. That is, the term “warm press forming” is contrasted withthe term “hot press forming” referring to forming a steel sheet into acertain shape after heat-treating the steel sheet at a temperaturehigher than the austenite single phase region.

In the present disclosure, the warm press forming includes a heattreatment process and a forming process and may be performed in theorder of a heat treatment process and a forming process or the order ofa forming process and a heat treatment process.

The inventors have found that when a member (component) is manufacturedthrough a warm press forming process, the elongation of the member canbe improved by properly adjusting the composition, microstructure, andheat treatment temperature of the member, and have invented the presentinvention based on the knowledge.

In a hot press forming process of the related art, a steel sheet isheated to a temperature higher than the austenite single phase region soas to form martensite as a main microstructure phase of the steel sheetwhile suppressing the formation of ferrite, and then the steel sheet isformed to have a desired shape and rapidly cooled to a temperature lowerthan a Mf (martensite finishing point), so as to form a high-strengthmember having martensite as a main microstructure phase.

However, in a warm press forming method of the present disclosure, asteel sheet is heat-treated at a temperature lower than the austenitesingle phase region and is subjected to a forming process and a coolingprocess. The warm press forming method of the present disclosure isproposed based on the knowledge that if a steel sheet is heated andmaintained at a temperature lower than the austenite single phaseregion, elements such as C and Mn are concentrated in austenite formedin grains or grain boundaries, and thus the austenite can be stable atroom temperature after the forming process and the cooling process.

Hereinafter, a steel sheet for warm press forming will be described indetail according to an embodiment of the present disclosure.

(Steel Sheet for Warm Press Forming)

First, the composition of the steel sheet for warm press forming will bedescribed in detail (hereinafter, concentrations are expressed in weight% unless otherwise specified).

Carbon (C): 0.01% to 0.5%

Carbon (C) is an element for increasing the strength of the steel sheet,and the concentration of carbon (C) is properly adjusted to ensure theformation of retained austenite in the steel sheet. If the concentrationof carbon (C) is less than 0.01%, the strength of the steel sheet maynot be sufficient, and it may be difficult to maintain 3 volume % ormore of retained austenite in the steel sheet during a warm pressforming process. Therefore, 0.01% or more (preferably, 0.05% or more) ofcarbon (C) is included in the steel sheet. If the concentration ofcarbon (C) is greater than 0.5%, it may be difficult to cold-roll thesteel sheet after the steel sheet is hot-rolled, and due to excessivelyhigh strength of the steel sheet, it may be difficult to obtain adesired elongation of the steel sheet. In addition, the weldability ofthe steel sheet may be lowered. Therefore, 0.5% or less (preferably,0.4% or less, and more preferably, 0.3% or less) of carbon (C) isincluded in the steel sheet.

Silicon (Si): 3.0% or Less (Excluding 0%)

Silicon (Si) functions as a deoxidizer during a steel making process andsuppresses the formation of carbides during a heat treatment process. Ifthe concentration of silicon (Si) is greater than 3%, it may bedifficult to plate the steel sheet. Thus, the concentration of silicon(Si) in the steel sheet may be 3% or less (preferably, 2.5% or less, andmore preferably, 2% or less).

Aluminum (Al): 3.0% or Less (Excluding 0%)

Aluminum (Al) removes oxides during a steel making process, and thus aclean steel sheet may be obtained. In addition, like silicon (Si),aluminum (Al) suppresses the formation of carbides during a heattreatment process. If the concentration of aluminum (Al) is high, atwo-phase region is extended, and thus the temperature range of theannealing process is widened. However, if the concentration of aluminum(Al) is greater than 3%, it may be difficult to plate the steel sheet,and the manufacturing cost of the steel sheet may be increased.Therefore, the concentration of aluminum (Al) in the steel sheet is setto be 3% or less (preferably, 2.5% or less, and more preferably, 2.0% orless.

Manganese (Mn): 3% to 15%

Manganese (Mn) has an important function in the embodiment of thepresent disclosure. Manganese (Mn) functions as a solid solutionstrengthening element and lowers the Ms (martensite start point)temperature for improving the stability of austenite at roomtemperature. In addition, since manganese (Mn) lowers the Ac1 and Ac3temperatures, manganese (Mn) has an important function in a warm pressforming process of the embodiment of the present disclosure.Furthermore, manganese (Mn) diffuses into austenite during a heattreatment performed at a temperature within the range of Ac1 to Ac3 in awarm press forming process, and thus the stability of the austenite maybe further improved at room temperature. If the concentration ofmanganese (Mn) in the steel sheet is less than 3%, these effects may notbe sufficiently obtained. Thus, the concentration of manganese (Mn) inthe steel sheet may be 3% or greater (preferably, 4% or greater, andmore preferably, 5% or greater). However, if the concentration ofmanganese (Mn) is greater than 15%, the manufacturing cost of the steelsheet is increased, and the amount of retained austenite may be toolarge. In this case, although the elongation of the steel sheet isincreased, the strength of the steel sheet may be insufficient.Therefore, the concentration of manganese (Mn) in the steel sheet may be15% or less (preferably, 13% or less, and more preferably 11% or less).

Phosphorus (P): 0.0001% to 0.1%

Like silicon (Si), phosphorus (P) suppresses the formation of carbideswhen martensite is heat-treated. However, in the case that the amount ofphosphorus (P) is excessive, the weldability and grain boundarycharacteristics of the steel sheet may be deteriorated. Therefore, theupper limit of the concentration of phosphorus (P) may be set to be0.1%. In addition, since manufacturing costs increase to maintain theconcentration of phosphorus (P) at a level lower than 0.0001%, the lowerlimit of the concentration of phosphorus (P) may be set to be 0.0001%.

Sulfur (S): 0.0001% to 0.03%

Sulfur (S) exists in the steel sheet as an impurity lowering theductility and weldability of the steel sheet. Such effects are not largeif the concentration of sulfur (S) is 0.03% or less, the upper limit ofthe concentration of sulfur (S) is set to be 0.03%. Since manufacturingcosts increase to maintain the concentration of sulfur (S) at a levellower than 0.0001%, the lower limit of the concentration of sulfur (S)is set to be 0.0001%.

Nitrogen (N): 0.03% or Less (Excluding 0%)

Nitrogen (N) exists in the steel sheet as an impurity. In the steelsheet, nitrogen (N) forms nitrides which improve resistance to delayedfractures caused by hydrogen. If the concentration of nitrogen (N) isgreater than 0.03%, a steel slab may become sensitive to cracks during acontinuous casting process, and pores may be easily formed in the steelslab. Therefore, the upper limit of the concentration of nitrogen (N) isset to be 0.03% (preferably 0.02%, and more preferably, 0.01%).

In addition to the above-mentioned elements, the steel sheet of theembodiment of the present disclosure may further include: at least oneof chromium (Cr), molybdenum (Mo), and tungsten (W) as an elementimproving hardenability; at least one of titanium (Ti), niobium (Nb),zirconium (Zr), and vanadium (V) as a precipitation strengtheningelement; at least one of copper (Cu) and nickel (Ni) as an elementimproving strength; boron (B) as an element improving grain boundarystrengthening and hardenability; and at least one of antimony (Sb) andtin (Sn) as an element improving plating characteristics.

Combination of at Least One of Chromium (Cr), Molybdenum (Mo), andTungsten (W): 0.001% to 2.0%

Chromium (Cr), molybdenum (Mo), and tungsten (W) improve hardenabilityand precipitation strengthening, and thus increase the strength of thesteel sheet. If the concentration of chromium (Cr), molybdenum (Mo), ortungsten (W) is lower than 0.001%, sufficient hardenability andprecipitation strengthening may not be obtained, and if theconcentration of chromium (Cr), molybdenum (Mo), or tungsten (W) isgreater than 2.0%, such effects may not be further obtained althoughmanufacturing costs increase. Therefore, the upper limit of theconcentration of chromium (Cr), molybdenum (Mo), or tungsten (W) is setto be 2.0%.

Combination of at Least One of Titanium (Ti), Niobium (Nb), and Vanadium(V): 0.001% to 0.4%

Titanium (Ti), niobium (Nb), and vanadium (V) are effective in improvingthe strength, grain refinement, and heat-treatment characteristics ofthe steel sheet. If the concentration of titanium (Ti), niobium (Nb), orvanadium (V) is lower than 0.001%, such effects may not be obtained, andif the concentration of titanium (Ti), niobium (Nb), or vanadium (V) isgreater than 0.4%, manufacturing costs increase. Therefore, theconcentration of titanium (Ti), niobium (Nb), or vanadium (V) may be setto be within 0.001% to 0.4%.

Combination of at Least One of Copper (Cu) and Nickel (Ni): 0.005% to2.0%

Copper (Cu) forms a fine Cu precipitate to improve the strength of thesteel sheet. If the concentration of copper (Cu) is lower than 0.005%,the strength of the steel sheet may not be sufficiently increased, andif the concentration of copper (Cu) is greater than 2.0%, theprocessability of the steel sheet may be deteriorated. Therefore, it maybe preferable that the concentration of copper (Cu) be set to be within0.005% to 2.0%. Nickel (Ni) improves the strength and heat-treatmentcharacteristics of the steel sheet. However, if the concentration ofnickel (Ni) is less than 0.005%, such effects may not be obtained, andif the concentration of nickel (Ni) is greater than 2.0%, manufacturingcosts increase. Therefore, the concentration of nickel (Ni) may be setto be within 0.005% to 2.0%.

Boron (B): 0.0001% to 0.01%

Boron (B) improves the hardenability of the steel sheet, and although asmall amount of boron (B) is added to the steel sheet, the strength ofthe steel sheet may be markedly increased through a heat treatment. Inaddition, boron (B) enhances grain boundaries and thus suppresses grainboundary embrittlement of the steel sheet having a large amount ofmanganese (Mn). However, if the concentration of boron (B) in the steelsheet is less than 0.0001%, such effects may not be obtained. Inaddition, if the concentration of boron (B) is greater than 0.01%, sucheffects may not be further obtained, and the high-temperatureprocessability of the steel sheet may be deteriorated. Therefore, theupper limit of the concentration of boron (B) may be set to be 0.01%.

Combination of at Least One of Antimony (Sb) and Tin (Sn): 0.0001% to1.0%

Antimony (Sb) and tin (Sn) may be concentrated on the surface and grainboundaries of the steel sheet. Thus, antimony (Sb) and tin (Sn) mayprevent the manganese (Mn) included in the steel sheet in a highconcentration from concentrating on the surface of the steel sheet andgenerating oxides during an annealing process of the steel sheet.Therefore, the steel sheet may be easily plated in a plating process.However, if the concentration of antimony (Sb) or tin (Sn) in the steelsheet is less than 0.0001%, such effects may not be obtained. Inaddition, if the concentration of antimony (Sb) or tin (Sn) is greaterthan 1.0%, the high-temperature processability of the steel sheet may bedeteriorated. Therefore, the upper limit of the concentration of theantimony (Sb) or tin (Sn) may be set to be 1.0%.

The steel sheet may include iron (Fe) and inevitable impurities as theremainder of constituents. However, the steel sheet may further includeother elements as well as the above-mentioned elements.

In the embodiment of the present disclosure, the steel sheet for warmpress forming may be one of a hot-rolled steel sheet, a cold-rolledsteel sheet, and a plated steel sheet. However, the steel sheet of thepresent disclosure is not limited but may be any kind of steel sheet.The plated steel sheet may be a Zn-based plated steel sheet or anAl-based plated steel sheet.

The steel sheet for warm press forming may have a main microstructureformed by 30 volume % or more of martensite, bainite, or a combinationthereof. If the steel sheet has a main microstructure formed by lessthan 30 volume % of martensite, bainite, or a combination thereof,austenite may not be sufficiently formed in the steel sheet during aheat treatment of a warm press forming process, and the strength of thesteel sheet may not be sufficiently high.

Hereinafter, a method of manufacturing a steel sheet for warm pressforming will be described in detail according to an embodiment of thepresent disclosure.

(Method of Manufacturing Steel Sheet for Warm Press Forming)

A steel slab including the above-described composition is heated to1000° C. to 1400° C., and is hot-rolled. If the heating temperature ofthe steel slab is lower than 1000° C., the microstructure of the steelslab formed after a continuous casting process may not be sufficientlyhomogenized, and if the heating temperature of the steel slab is higherthan 1400° C., manufacturing costs may be increased.

Thereafter, the steel slab is subjected to a finish hot rolling processat a temperature within a temperature range of Ar3 to 1000° C. to form ahot-rolled steel sheet. If the process temperature of the finish hotrolling process is lower than Ar3, two-phase rolling may occur to causea mixed grain size distribution and lower processability. On thecontrary, if the process temperature of the finish hot rolling processis greater than 1000° C., the grains of the steel slab may be coarsened,and a large amount of oxide scale may be generated.

Thereafter, the hot-rolled steel sheet is coiled at a temperature higherthan Ms but equal to or lower than 800° C. If the hot-rolled steel sheetis coiled at a temperature equal to or lower than Ms, a large load maybe applied to a hot-rolling coiler, and if the hot-rolled steel sheet iscoiled at a temperature higher than 800° C., the thickness of an oxidelayer of the hot-rolled steel sheet may be increased.

The hot-rolled steel sheet manufactured as described above may be usedin a warm press forming process or may be additionally treated through apickling process. Furthermore, after the hot-rolled steel sheet ispickled, the steel sheet may be plated with a Zn-based material or anAl-based material, and then the plated steel sheet may be used in a warmpress forming process.

In addition, the hot-rolled steel sheet may be subjected to a picklingprocess and a cold rolling process to produce a cold-rolled steel sheet.The pickling process may be performed according to a general method, andthe reduction ratio of the cold rolling process is not limited. Forexample, the reduction ratio of the cold rolling process may be selectedfrom general values used in the related art.

For example, before the hot-rolled steel sheet is cold-rolled, thehot-rolled steel sheet may be batch-annealed. Since the hot-rolled steelsheet manufactured as described above has a high degree of strength, thehot-rolled steel sheet may be batch-annealed to reduce the strengththereof and thus to reduce the load of the cold rolling process. Thatis, the cold rolling processability of the hot-rolled steel sheet may beimproved. It may be preferable that the batch annealing be performedwithin the temperature range of Ac1 to Ac3. If the process temperatureof the batch annealing is lower than Ac1, the strength of the hot-rolledsteel sheet may not be sufficiently lowered. On the contrary, if theprocess temperature of the batch annealing is higher than Ac3,manufacturing costs may be increased, and a large amount of martensitemay be formed in the hot-rolled steel sheet when the hot-rolled steelsheet is slowly cooled after the batch annealing. In this case, thestrength of the hot-rolled steel sheet may not be sufficiently lowered.After the batch annealing, the hot-rolled steel sheet may be cold-rolledto produce a cold-rolled steel sheet.

The cold-rolled steel sheet may be treated through a continuousannealing process to produce an annealed steel sheet. Process conditionsof the continuous annealing process are not limited. For example,preferably, the continuous annealing process may be performed at atemperature within the temperature range of 700° C. to 900° C. If theprocess temperature of the continuous annealing process is lower than700° C., the steel sheet may not be sufficiently recrystallized. If theprocess temperature of the continuous annealing process is greater than900° C., manufacturing costs may be increased, and processability may belowered. The annealed steel sheet may be plated through a Zn—Nielectroplating process to produce a Zn—Ni electroplated steel sheet.

Alternatively, the cold-rolled steel sheet may be plated with a Zn-basedmaterial or an Al-based material so as to improve the corrosionresistance and thermal resistance of the cold-rolled steel sheet.Heat-treatment and Zn-plating conditions for the cold-rolled steel sheetare not limited. For example, the cold-rolled steel sheet may be hot-dipgalvanized to produce a product known as a GI (galvanized iron) sheet ormay be hot-dip galvannealed to produce a product known as a GA(galvannealed) steel sheet. In addition, heat-treatment and Al-platingconditions for the cold-rolled steel sheet are not limited. For example,conditions generally used in the related art may be used.

Hereinafter, a warm-pressed member manufactured through a warm pressforming process using the above-described steel sheet will be describedaccording to an embodiment of the present disclosure.

(Warm-Pressed Member)

In the embodiment of the present disclosure, the warm-pressed memberincludes the above-described composition of the steel sheet for warmpress forming. The microstructure of the warm-pressed member mayinclude: 3 volume % to 50 volume % of retained austenite; and at leastone of ferrite, martensite, tempered martensite, and bainite as aremainder.

If the volume fraction of retained austenite is lower than 3%, thewarm-pressed member may not have an ultra high degree of strength and ahigh degree of elongation desired in the embodiment of the presentdisclosure. On the contrary, if the volume fraction of retainedaustenite is higher than 50%, it may be difficult to produce thewarm-pressed member because large amounts of C and Mn have to beincluded in the warm-pressed member. In addition to the retainedaustenite, the microstructure of the warm press forming may include atleast of ferrite, martensite, tempered martensite, and bainite.

Ferrite may be formed in the warm-pressed member during a heat treatmentof a warm press forming process (described later) or may be partiallyformed before the heat treatment. Preferably, the fraction of ferrite inthe warm-pressed member may be 30% or less. If the fraction of ferriteis greater than 30%, the warm-pressed member may not have sufficientstrength.

Martensite may be formed in the warm-pressed member during a heattreatment of a warm press forming process or may be partially formedbefore the heat treatment. At this time, carbides may be partiallyformed in the martensite. The fraction of martensite in the warm-pressedmember may be within the range of 50% to 95%. If the fraction ofmartensite is lower than 50%, the warm-pressed member may not havesufficient strength, and if the fraction of martensite is greater than95%, retained austenite may not be sufficient included in thewarm-pressed member.

Hereinafter, a method of manufacturing a warm-pressed member will bedescribed in detail according to an embodiment of the presentdisclosure.

(Method of Manufacturing Warm-Pressed Member)

In the embodiment of the present disclosure, a warm press forming methodis used to form a member having a high degree of elongation. Theinventors have researched into a method of manufacturing a member havingdesired properties through a warm press forming process based on theknowledge that a desired degree of thermal resistance of a plating layercan be guaranteed if a heat treatment is performed at a temperaturelower than Ac3. As a result, it is found that if a steel sheet havingthe above-mentioned composition is heat-treated at a temperature lowerthan Ac3, the steel sheet can have retained austenite.

That is, it is found that if a steel sheet including manganese (Mn) isproperly subjected to a hot rolling process, and/or a cold rollingprocess, and an annealing process, the steel sheet can have amicrostructure of 5 μm or less before a heat treatment. In addition, itis found that if a steel sheet includes sufficient amounts of martensiteand/or bainite before a heat treatment, nano-sized lath grains of themartensite and/or bainite are converted into austenite or manganese (Mn)and carbon (C) stabilize the austenite during a heat treatment of a warmpress forming process to form a stable austenite structure even at roomtemperature. As described above, it may be preferable that the mainmicrostructure of a steel sheet for warm press forming be formed by 30%or more of martensite, bainite, or a combination thereof. If thefraction of martensite, bainite, or a combination thereof in a steelsheet is low, a sufficient amount of austenite may not be formed in thesteel sheet during a heat treatment of a warm press forming process, andthe steel sheet may not have a desired degree of strength.

A member manufactured based on the above-mentioned knowledge has 3volume % or more of retained austenite and thus good elongation.

In the method of manufacturing a warm-pressed member, a steel sheetmanufactured as described above is subjected to a warm press formingprocess. In the warm press forming process, a forming process may beperformed after or before a heat treatment.

The heat treatment of the warm press forming process may be performed byheating the steel sheet to a temperature within a temperature range ofAc1 to Ac3 with a heating rate of 1° C./sec to 1000° C./sec. Then, thesteel sheet is maintained at the temperature within the temperaturerange for 1 second to 10000 seconds.

If the heating rate is lower than 1° C./sec, manufacturing costs may beincreased, and productivity may be lowered. Therefore, the lower limitof the heating rate may be set to be 1° C./sec. Although the heatingrate is greater than 1000° C./sec, the effect of the heat treatment isnot increased but an excessive amount of heating equipment may berequired. Therefore, the upper limit of the heating rate may be set tobe 1000° C./sec.

The temperature range of Ac1 to Ac3 is important to guarantee theformation of retained austenite. If the heat treatment is performed at atemperature lower than Ac1, austenite may not be formed in grains orgrain boundaries of martensite or bainite, and thus retained austenitemay not be obtained. Therefore, the heat treatment may be performed at atemperature equal to or greater than Ac1 (preferably, Ac1+10° C. andmore preferably, Ac1+20° C.). If the heat treatment is performed at atemperature greater than Ac3, carbon (C) and manganese (Mn) may not besufficiently concentrated on austenite, and thus the stability ofretained austenite may be low. That is, a sufficient amount of retainedaustenite may not be obtained, and thus the elongation of the steelsheet may not be sufficient even though the strength of the steel sheetmay be increased. Therefore, the upper limit of the temperature range ofthe heat treatment may be set to be Ac3 (preferably, Ac3-10° C., andmore preferably, Ac3-20° C.)

If the steel sheet is maintained within the heat-treatment temperaturerange for a period of time longer than 10000 seconds, productivity maybe decreased, and martensite may disappear to lower the strength of thesteel sheet. Therefore, the upper limit of the period of time may be setto 10000 seconds.

Thereafter, the steel sheet is warm-pressed and cooled. At this time,the cooling rate is not limited. For example, it may be preferable thatthe cooling rate range from 1° C./sec to 1000° C./sec. If the coolingrate is lower than 1° C./sec, productivity may be lowered, andadditional equipment may be used to control the cooling rate. Therefore,manufacturing costs may be increased. If the cooling rate is greaterthan 1000° C./sec, additional equipment may be used to rapidly cool thesteel sheet, and the microstructure of a warm-pressed member formed ofthe steel sheet may not be appropriate.

MODE FOR INVENTION

Hereinafter, examples of the present disclosure will be described indetail. The following examples are for illustrative purposes and are notintended to limit the scope of the present disclosure.

Examples

Steel slabs having compositions as shown in Table 1 were produced by avacuum melting process, and the steel slabs were reheated in a heatingfurnace at 1200° C. for 1 hour and were hot-rolled. The hot rolling ofthe steel slabs were finished at 900° C., and the hot-rolled steel slabs(hot-rolled steel sheets) were cooled at 680° C. in a furnace. A warmpress forming process was performed on the hot-rolled steel sheets undersimulated conditions.

Meanwhile, the hot-rolled steel sheets were pickled and then a coldrolling process was performed on the pickled hot-rolled steel sheetswith a cold rolling reduction ratio of 50% so as to produce cold-rolledsteel sheets. Particularly, steel sheets M and N were treated through abatch annealing process after the cold rolling process. In the batchannealing process, the steel sheets M and N were heated at a heatingrate of 30° C./h and maintained at 600° C. for 10 hours. Thereafter, thesteel sheets M and N were cooled at a cooling rate of 30° C./h. Acontinuous annealing process was performed on the other steel sheetsinstead of the batch annealing process. The continuous annealing processwas performed at 780° C.

In addition, the picked hot-rolled steel sheets and the cold-rolledsteel sheets were plated through a zinc (Zn) or aluminum (Al) platingprocess so as to produce plated steel sheets. Specifically, in the zinc(Zn) or aluminum (Al) plating process, the steel sheets were annealed at780° C. and then were dipped in a zinc (Zn) or Aluminum (Al) platingbath.

The pickled hot-rolled steel sheets, the cold-rolled steel sheets, andthe plated steel sheets were treated under simulated heat treatmentconditions of the warm press forming process. The heat treatmentconditions are shown in Table 2 below. The heating rate of the heattreatment was 3° C./sec.

Tension test specimens of the steel sheets processed through the warmpress forming process under simulated conditions were prepared accordingto JIS Z 2201 #5, and mechanical properties of the tension testspecimens were measured. In addition, the fraction of retained austenitein each of the steel sheets was measured by an X-ray diffraction test.In detail, the fraction of retained austenite were calculated by a 5peak method expressed in Equation 1 using the areas of austenite (200),(220), and (311) peaks and the areas of ferrite (200) and (211) peaksobtained in the X-ray diffraction test. In Equation 1, V_(γ) refers toan austenite fraction, I_(α) refers to a ferrite peak area, and I_(γ)refers to an austenite peak area.

$\begin{matrix}{V_{\gamma}^{XRD} = \frac{\begin{matrix}\begin{matrix}{\left\lbrack {{{1/2.19}\left( {I_{\alpha}^{200}/I_{\gamma}^{200}} \right)} + 1} \right\rbrack + \left\lbrack {{{1/1.35}\left( {I_{\gamma}^{220}/I_{\alpha}^{220}} \right)} + 1} \right\rbrack +} \\{\left\lbrack {{{1/1.5}\left( {I_{\alpha}^{200}/I_{\gamma}^{311}} \right)} + 1} \right\rbrack + \left\lbrack {{{1/1.12}\left( {I_{\alpha}^{211}/I_{\gamma}^{200}} \right)} + 1} \right\rbrack +}\end{matrix} \\{\left\lbrack {{{1/0.7}\left( {I_{\alpha}^{211}/I_{\gamma}^{220}} \right)} + 1} \right\rbrack + \left\lbrack {{{1/0.78}\left( {I_{\alpha}^{211}/I_{\gamma}^{311}} \right)} + 1} \right\rbrack}\end{matrix}}{6}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Mechanical properties and retained austenite fractions of the steelsheets measured as described above are shown in Table 2 below.

TABLE 1 Steel sheets C Si Mn P S Al N Others Notes A 0.08 0.1 5.1 0.0140.003 0.04 0.004 — *IS B 0.07 0.1 7.0 0.012 0.004 0.03 0.003 — IS C 0.070.1 10.0 0.014 0.003 0.02 0.004 — IS D 0.15 1.56 6.1 0.010 0.005 2.290.004 — IS E 0.16 0.1 5.0 0.014 0.003 0.04 0.004 B: 0.0026 IS F 0.31 0.15.0 0.014 0.003 0.03 0.004 Ti: 0.02 IS G 0.32 1.6 5.0 0.014 0.003 0.040.004 Nb: 0.03 IS H 0.16 0.1 6.9 0.013 0.003 0.03 0.003 Zr: 0.05 IS I0.30 0.1 6.9 0.013 0.003 0.03 0.003 W: 0.04 IS J 0.30 0.7 6.9 0.0130.003 0.03 0.003 Cr: 0.3 IS K 0.29 0.6 7.1 0.015 0.004 0.05 0.005 Mo:0.05 IS L 0.03 0.1 9.1 0.013 0.003 0.02 0.004 Cu: 0.05 IS M 0.04 0.1 9.50.015 0.003 0.05 0.004 Ni: 0.11 IS N 0.15 0.1 9.9 0.014 0.002 0.01 0.004V: 0.05 IS O 0.14 0.1 9.8 0.015 0.005 0.11 0.005 Sb: 0.05 IS P 0.02 0.114.2 0.014 0.003 0.04 0.004 Sn: 0.04 IS Q 0.23 0.2 1.3 0.011 0.003 0.030.005 Cr: 0.17, **CS Ti: 0.03, B: 0.0026 R 0.28 1.5 1.5 0.010 0.003 0.020.007 Nb: 0.05, CS B: 0.003 *IS: Inventive steel, **CS: Comparativesteel

TABLE 2 Heat treatment conditions Mechanical Retained Cooling propertiesaustenite Steel Product Temp. Time rate TS El fraction sheets types (°C.) (sec) (° C./sec) (MPa) (%) (%) Notes A CR 700 300 45 1054 17 7.3 *IEZn 700 300 5 1031 18 7.8 IE Zn 850 300 45 1201 6 2.1 CE B CR 650 300 451124 20 9.0 IE C HR 500 300 45 1356 15 13.4 IE Al 600 300 45 1330 1920.6 IE D CR 740 300 45 1042 31 16.9 IE E CR 700 300 45 1127 14 11.6 IEF CR 700 300 45 1297 13 9.6 IE G Zn 700 300 45 1102 27 10.9 IE H CR 600300 45 1121 20 16.7 IE Zn 650 300 5 1249 26 16.8 IE I CR 650 300 45 120628 18.8 IE J CR 650 300 45 1189 23 28.1 IE K CR 650 300 45 1236 21 25.6IE L Zn 500 300 45 1052 16 6.9 IE M CR 500 300 45 1063 18 8.1 IE N CR500 300 45 1491 18 18.3 IE CR 600 300 45 1428 17 22.8 IE O CR 600 300 451436 17 21.5 IE P Zn 550 300 45 1015 26 31.4 IE Q Al 600 300 45 541 220.5 **CE Al 900 300 45 1629 6 0.3 CE R CR 750 300 45 786 21 1.7 CE CR850 300 45 1899 7 0.7 CE *IE: Inventive Example, **CE: ComparativeExample

Products produced using steel sheets A to P having compositionsaccording to the present disclosure have retained austenite fractions of3% or greater and good elongation. However, products produced usingcomparative steel sheets Q and R have retained austenite fractions ofless than 3% regardless of heat treatment conditions and have poorelongation.

When the steel sheet A was heat-treated at 850° C. higher than Ac3 inthe warm press forming process, the strength of the steel sheet A wassufficiently high but the elongation thereof was decreased because ofinsufficient amount of retained austenite.

1. A warm-pressed member comprising, by weight %, C: 0.01% to 0.5%, Si:3.0% or less (excluding 0%), Mn: 3% to 15%, P: 0.0001% to 0.1%, S:0.0001% to 0.03%, Al: 3.0% or less (excluding 0%), N: 0.03% or less(excluding 0%), and the balance of Fe and inevitable impurities, whereinafter a warm press forming process and a cooling process, thewarm-pressed member has a microstructure comprising: 5 volume % to 50volume % of retained austenite; and at least one of ferrite, martensite,tempered martensite, and bainite as a remainder.
 2. The warm-pressedmember of claim 1, wherein the warm-pressed member has a tensilestrength of 1000 MPa or greater and an elongation of 10% or greater. 3.The warm-pressed member of claim 1, further comprising 0.001% to 0.4% ofat least one selected from the group consisting of Ti, Nb, Zr, and V. 4.The warm-pressed member of claim 1, further comprising 0.005% to 2.0% ofat least one of Cu and Ni.
 5. The warm-pressed member of claim 1,further comprising 0.0001% to 1.0% of at least one of Sb and Sn.
 6. Thewarm-pressed member of claim 1, further comprising 0.0001% to 0.01% ofB.